1. Field of the Invention
The present invention relates to an air-fuel ratio control device for internal combustion engines.
2. Description of the Prior Art
The components of the gas burned and exhaust from an internal combustion engine and the engine torque are closely related to the air-fuel ratio (A/F) of the carburetor supplying the air-fuel mixture to the engine, as shown in FIG. 1. Therefore, while automobile exhaust can theoretically be cleaned by a variety of methods, there are only three main practical methods from the standpoint of the A/F ratio. These are:
(1) Setting the air-fuel ratio to the rich mixture region where the emission of nitrogen oxides (NOx) is small, as shown by A in FIG. 1, installing a cleaner such as catalyst in the exhaust pipe, and supplying additional air by a secondary air feeding device, in order to remove unburnt carbon monoxide (CO) and hydrocarbons (HC),
(2) Operating the engine in the lean mixture region where the emission of NOx, CO, and HC is small, as indicated by B in FIG. 1, and
(3) Installing a three-way catalyst in the exhaust pipe, setting the A/F ratio near the stoichiometric A/F ratio, as indicated by C in FIG. 1, and removing the CO, HC, and NOx simultaneously by the three-way catalyst.
Recently, automotive designers have been required not only to produce engines with cleaner exhaust but also, from the viewpoint of resource conservation, to produce engines with lower fuel consumption rates. To simultaneously clean the exhaust and reduce the fuel consumption rate, it is advantageous to operate the engine in the lean mixture region. In the lean mixture region, however, misfires easily occur. Individual manufacturing differences and aging of the engine and related parts (carburetors and the like) make it almost impossible for engines to be operated in the lean mixture region near where misfires would take place. In practice engines are operated in a rich, stable region, i.e., a region where the A/F ratio is about twice as dense as the misfire-prone region. This is a problem, therefore, in purifying the exhaust and saving resources.
As shown in FIG. 1, the minimum fuel consumption rate is achieved when the engine is operated at a lean A/F ratio just above the misfire region. The variation in combustion (variation of torque in this embodiment, as will be mentioned later) is related to the A/F ratio and increases rapidly on approaching the misfire region.